Mechanical seal assemblies are used to prevent fluids (liquids or gases) from leaking along a rotatable shaft from a relatively higher pressure region in an apparatus in which the shaft is mounted to a relatively lower pressure region. Such seal assemblies typically incorporate a non-rotatable seal ring which interacts with a rotatable seal ring connected around the shaft. Both the rotatable and non-rotatable seal rings are provided with sealing faces which oppose one another in sliding, mating relationship as they rotate relative to one another. Generally, the shaft seal ring is constructed of a relatively hard, non-weldable-type material, such as silicon carbide, tungsten carbide, or Stellite.RTM., so as to withstand operating conditions and provide a desirable seal life. The non-rotatable seal ring is commonly made of a softer material such as carbon, but can be a polymeric material, or ceramic, or the like.
Components which interact together, such as springs and/or bellows, and the like, are employed to urge the seal faces into contact with one another to provide the seal. A small portion of the fluid being sealed off creates a fluid film between the seal faces for cooling and lubricating them as they rotate relative to one another, thereby prolonging the operating life of the seals. In many instances, the clearance between the inner diameter of the non-rotatable seal ring and the shaft is small. In such cases, the fluid which seeps between the seal faces becomes trapped between the carbon ring and the rotating shaft. When relatively high temperatures are present, it is possible that the fluid may undergo thermal degradation, for example, producing coke when the fluid is oil, thereby generating highly abrasive particles. Such particles can damage the seal components, thereby reducing the effective life of the seal.
In many instances, mechanical seal assemblies are used in environments in which the seals are subjected to large temperature excursions. For example, when a mechanical seal is used in equipment which operates at high temperatures, e.g., up to 500.degree. F., or hotter, the seal components heat up to these high temperatures as the equipment heats up. Commonly, one or more elastomeric O-rings are incorporated into a mechanical seal assembly to inhibit the flow of fluids between the mechanical seal assembly and the supporting structure of the apparatus. Because the various components of the mechanical seal and the support structure in which it is located are commonly constructed of different materials which have different thermal coefficients of expansion, as the seal heats up, the different seal components and the support structure expand at different rates and in different amounts.
In prior-art mechanical seal designs, typically such secondary seals are in direct contact with mechanical seal components which are moveable for positioning the non-rotatable seal ring. In such prior-art seals, where the secondary O-ring seal is required to move relative to the positioning components of the mechanical seal as the components expand during heat up, the use of such secondary seals can cause the mechanical seal to fail. Such failure results from the elastomeric O-ring's tendency to stick or bind to the seal component or components which it contacts. Deterioration of the O-ring from chemical attack and from exposure to high temperatures exacerbates the sticking and binding problem. Such binding can inhibit the movement of the seal components as they expand relative to each other and relative to the support structure. Inhibiting the movement of the seal components can result in the seal faces unseating, causing the seal to fail.
In addition to the problems associated with the sticking of elastomeric O-rings during heat up, such O-rings deteriorate at such high temperatures and, thus, have a limited expected useful life in a high-temperature environment. Furthermore, in many applications, the secondary seals are subject to exposure to various fluids (liquids or gases) which tend to attack the elastomer, causing it to deteriorate.
It is therefore desirable to provide to the art a mechanical seal that is of a design that does not incorporate an elastomeric O-ring seal that is required to move with the mechanical seal components as the seal is heated to operating temperature. The improved mechanical seal will have an enhanced reliability and longer useful life than prior-art seals.